Google it!
You are unlikely to find high-quality scholarly secondary sources through a general internet search, but this is still a useful step. Universities, museums, archives, libraries, and professional organizations have many websites and pages devoted to topics in the history of math and science (see the sidebar "Links to Some Collections of Online Resources"). Through sites like these, you may find citations or bibliographies of secondary sources, as well as collections of images and primary sources. You may also find lower-quality secondary sources, like non-scholarly books or articles from newspapers, magazines, or blogs. You may find useful or interesting ideas in sources like these, but you should never rely on them without further research.
Be careful! You will often find outright false information through a general internet search––always ask yourself who the creator of the content is, what their interests or agenda may be, and where they obtained their facts. You may also find misleading, oversimplified, or mischaracterized historical information websites maintained by amateurs or scholars whose primary area of expertise isn't the history of science or math. For tips on how to assess the reliability of online sources, see this guide: Evaluating Sources. The only way to be sure is to check their sources yourself and to compare their information with what you know from the scholarly secondary sources you found above.